Sulphate digestion process



F. J. SOWA SULPHATE DIGESTION PROCESS Filed May 18. 1961 sept. 1, 1964 United States Patent O 3,147,178 SULPHATE DIGESTIN PROCESS Frank il. Sowa, Cranford, NJ., assigner to St. Joe Paper Company, Jacksonviile, Fla., a corporation of Fiorida Filed May 18, 196i, Ser. No. 111,03@ 13 Claims. (El. 162-13) This invention relates to sulphate type wood pulping processes and to the manufacture of paper from pulps derived by such procesess. One aspect of the invention particularly relates to the production of wood pulp from resinous woods such as those of the southern pine variety.

Sulphate type wood pulping processes are well known and involve the digestive treatment of the wood in cooking liquors which contain caustic soda and appreciable quantities of sodium sulphide as the principal delignifying constituents of the liquors, the digestive treatment being carried out under elevated temperature and pressure conditions.

The invention described herein is concerned with the use of certain cooking liquor additives in such types of processes, and has as a principal object the provision of economical and practical methods by which the color and brightness properties of the sulphate pulps may be improved through the use of such cooking liquor additives.

In the kraft type of sulphate process the wood is intentionally undercooked to produce pulps of exceptional strength, the color of the pulps and the amount of pulp yielded from the wood being determined primarily by the nature of the cooking procedure. In most cases the cooking procedure followed is such that high yield pulps are obtained which are characteristically darker in color than the low yield kraft pulps, and past use of the high yield kraft pulps has been largely confined tol the production of coarse wrapping, packaging and industrial papers wherein the darker color and poor brightness characteristics of the pulps have not been considered objectionable. Current trends, however, are toward the greater use of kraft type paper in the packaging of consumer purchases, and the demand in such cases is for papers which are lighter in color and exhibit better brightness characteristics such as those developed by low yield kraft type cooking procedures.

In obtaining low yield kraft pulps, more drastic cooking conditions are employed in digestively treating the wood as by resorting to the use of greater amounts of chemical, and/ or higher cooking temperatures and/or longer cooking periods. Pulp yields in such cases normally run from about 45% to about 50% based on the oven dry weight of wood treated, whereas the pulp yielded by the wood when digesting conditions are designed to produce high yield kraft pulps normally are considered as exceeding 50% 0n the same basis. In producing bleachable grades of sulphate pulp even more drastic conditions are employed with corresponding greater losses in pulp yield by virtue of the more drastic conditions.

Kraft pulp producers, in many instances, recently have found it necessary to employ the more drastic digesting conditions so as to satisfy customer demands, and, accordingly, have found it necessary to either absorb or pass on to the customers the higher costs associated with the use of lower yield pulps. In such cases, and as a means of minimizing the amount of low yield pulp employed in forming the paper, advantage has been taken of customer specifications which require that only one side of the paper sheet meet minimum brightness values and of the conventional practice of adding the pulp to the wire of the paper machine by the primary and secondary stock addition technique. This technique is usually employed as a means for developing a better finish on one side of the paper sheet than the other, and when so employed involves, as is well known, rst the addition of a major por- 3,147,178 Patented Sept. l, 1964 "ice tion of the pulp employed in the formation of the sheet to the wire of the paper machine and then the addition of the balance of the pulp in a more highly rened condition to the Wire of the paper machine as a secondary stock addition. By adding a high yield darker colored kraft pulp to the wire as the primary stock addition and then employing the lower yield lighter colored sulphate type pulp as .the secondary stock addition, paper producers have been able to use substantial amounts of the darker colored higher yield pulps in the production of the paper sheet, and thus avoid making the sheets entirely from the lighter colored low yield pulps. The sheet of paper produced is a composite of both pulps and has the color and brightness characteristics on opposite sides of the sheet which are characteristic of the respective pulps.

The technique has also been employed -to advantage with lighter and brighter pulps derived by methods other than the more drastic sulphate type digesting conditions. In some instances special light pulp producing woods have been treated to form pulps which are employed in the formation of the compositesheets. In other cases different types of special digesting procedures have been employed to secure the lighter pulps which are utilized for the secondary stock. For example, some mills have employed a combination digestion process involving both a sulphate and a neutral sulphite cook. This, of course, is complicated and relatively uneconomical. In all of such cases, however, there is the problem of color-showthrough which results in a mottled effect on the lighter or secondary side of the sheet, and which is overcome only by employing such suiciently large amounts of the secondary stock as to insure against this result.

Accordingly, one object of the invention is the provision of methods for producing lighter colored high yield kraft type pulps which enable a reduction in the amount of secondary stock employed for the formation of composite sheets from pulps having different color characteristics.

A general object of the invention, however, is to provide new sulphate type processes of pulping wood.

It is an object of the invention to provide methods of improving sulphate type wood pulping processes in general so that the pulps derived by such processes are of better color.

Another object is to provide methods by which the cost of rening sulphate pulps may be decreased through the production of pulps which are stronger and thereby require less refining to develop their strength properties.

Another object is to provide methods by which freer sulphate type pulps may be produced, thus enabling faster paper machine operations and lower costs for the drying of the webs of paper produced thereon.

Another object is to improve sulphate type pulping processes so that more soap may be recovered from the black liquor when treating resinous woods.

Another object of the invention is to provide a practical and economical method of producing lighter colored paper which is a composite of pulps derived by separate sulphate pulping treatments.

Still another object of the invention is to provide improvements, in sulphate processes of producing pulp from wood, which facilitate the greater recovery of pulp from old woods.

A specic object of the invention is to improve sulphate type pulping processes of treating resinous wood particles so as to produce brighter and better colored pulps less expensively than, and without the need for, resorting to more drastic digesting conditions.

Another object of the invention is to provide methods for improving sulphate type wood pulping processes so that the pulps derived exhibit better color and brightness properties and which when embodied in mill operations 3 provide a secondary means for adding make-up chemical to the system.

These and other objects will be apparent from the following detailed disclosure and accompanying drawings wherein FIG. 1 schematically illustrates one aspect of the invention as embodied in the cooking procedure of a sulphate type wood pulping process; and

FIG. 2 schematically illustrates a kraft type pulping and paper producing operation embodying the invention.

Broadly, the invention contemplates the digestive treatment of wood in sulphate cooking liquor to which has been added a small amount of material selected from one or more of the compounds in the group generally denominated as the alkali metal silicates and silicic acid. The alkali metal silicates and the various forms of silicio acid are soluble in hot caustic solutions such as those encountered in sulphate type pulping processes. Silicic acid is readily soluble in hot caustic soda solutions and exists in various hydrated forms such as orthosilicic acid (H4Si04) and metasilicic (H2SiO3) as Well as in more dehydrated forms, such as H6Si207, H4Si206, H2Si205 and H4Si308, and, accordingly, unless otherwise expressly provided for, the expression silicic acid is used herein in its generic sense as having reference to t'ne various hydrated forms of silicic acid which are readily soluble in hot aqueous caustic soda solutions such as encountered in sulphate cooking processes. The alkali metal silicates, as is well known, are salts of silicic acid, and, accordingly, the term silicate is used herein in a like generic sense unless otherwise expressly used in reference to a salt of a specific form of silicic acid.

In practice a sodium silicate, such as sodium orthosilicate, sodium sesquisilicate or sodium metasilicate, is the preferred additive because of its relative inexpensiveness in todays market and for the added reason that it serves as a source of make-up chemical in conventional sulphate mill operations. Sodium silicate is available commercially with various ratios of Na2O2SiO2 such as, sodium orthosilicate having a ratio of NaZOzSiOg of 2:1, sodium metasilicate having a ratio of Na2OzSiO2 of 1:1, sodium sesquisilicate having a ratio of Na2O1SiOZ of 3:2, as Well as other commercial sodium silicate products having various ratios of Na2OzSiO2 such as 1:2, 1:3.2 etc. However, pulps which exhibit better color and brightness properties may be obtained by using other alkali metal silicates such as potassium silicate and lithium silicate, and, in general, it is contemplated that all of the various forms of the alkali metal silicates such as potassium orthosilicate, potassium metasilicate, lithium orthosilicate, and lithium metasilicate will function in a manner which, in this respect, is similar to sodium orthosilicate or sodium metasilicate. Commercially available potassium silicates having ratios of K2O:Si02 corresponding to those for sodium silicates are useful as are those having ratios of K2O:Si02 of 1:2.1, 1:22, 1:2.5 etc. In lieu of one or more of the alkali metal silicates, silicic acid, such as orthosilicic acid and metasilicic acid may be employed. Nevertheless, the various forms of silicic acid are more costly and lack the advantage of functioning as a source of make-up caustic in actual mill operations such as rcsults from the use of sodium silicate.

In carrying the invention into practice, a small amount of additive material, such as sodium orthosilicate or sodium metasilicate, or mixtures thereof is added to the sulphate type cooking liquor, either prior to its being charged to the digester or after being charged to the digester. The additive material may be added either in a solid form or as dissolved in a suitable solvent solution, such as an aqueous solution of caustic soda or water when appropriate. Solutions of one or more of the additives may be sprayed upon the chips as the chips are being charged to the digester.

I have found it convenient in actual kraft pulp mill operations, employing top loaded batch type digesters, to add the cooking liquor additives to the digester after the chips have been charged thereto and prior to the charging of the cooking liquor thereto. In this way the liquor, when being charged to the digester, tends to more or less uniformly distribute the additives throughout the digester. Other methods of charging the additives will be apparent to those skilled in the art, and it is contemplated that the additives may be incorporated in the cooking liquor after the Wood has been partly digested.

As previously indicated, only small amounts of the additives should be employed. Their effectiveness apparently being related to their equivalent SiOg content, and the amount of wood digestively treated. Accordingly, it is convenient to refer to the quantities or amounts em- TABLE I [Average results of kraft cooks] RunA RunB RunC RunD RunEz SiOz/ S102/ S102/ S102/ SiOg/ Control Wood 1 Control Wood 1 Control Wood 1 Control Wood 1 Control Wood 1 No. of Cooks 16 6 10 6 6 4 6 6 6 6 Total Yield 3.- 52. 4 52.0 5l. 0 51.2 51.0 50. 7 5l. 0 51. 1 51.2 52.0 Screen Yield 4- 38. 7 38. 2 38. 4 38. 0 38. 8 38. 4 3B. 8 38. 8 41. 8 43. 4 Screened Pulp Brig 1tness 5 22. 6 22. 9 23.1 23. 7 22. 2 23. 8 22. 2 23. 0 22. 2 23. 2 Screened Pulp ClgNo 1 11. 4 11. 8 10. 8 11.4 10. 6 10. 2 10. 6 10.2 10.5 10.0 Beet Pulp (2o Min.)

Freeness 7 600 595 535 575 595 555 595 565 5115 430 Beat Pulp (20 Mln.)

Mullen E- 31. 0 30. 8 29. 8 33.3 30. 0 31.0 30.0 32. 5 30. 0 3-1. 0 Beat Pulp (3 Min.)

Freencss 7- 420 420 435 425 450 380 450 370 330 100 Beat Pulp Mullen 1 38.5 38. 3 37.0 40. 3 37.3 39.5 37.3 40. 8 38. 0 42.0 Beat Pulp Tear 1 165 165 153 164 14D 164 152 150 120 Freeness at rn ull e n strength 40.0 880 380 320 430 370 305 370 380 Color Remarks Slightly Lighter Lighter Than Consicerably Consic erahly Consieerably Than Control Control Lighter Lighter Lighter 1 Additive-towood ratio. 1 Old wood used in this run.

3 Percent weight ratio of pulp yielded to wood charged to digestcr (oven dry basis). 4 Percent weight ratio o1 pulp passing a 0% dat pulp screen to Wood charged to digester (oven dry basis). 1 Percent reflectance ot 460 millimicron Wave lengths as compared to MgO standard white body having 100% rcllectnncc.

6 Roe C12N o. 7 Canadian standard.

1 TAPPI Standard Burstlng Strength, T403 11i-53 as corrected Aug. 1953 (lbs./in.2). 1 TAPPI Standard, T414 rn-49 as corrected Sept. 1949 (grams).

U ployed in terms of an additive-to-wood ratio which should be understood as being reported herein as a percentage weight ratio of the additive (such as sodium orthosilicate, sodium metasilicate, etc. or mixtures of one or more of the additives), expressed as the equivalent weight of SiOz, to the bone dry (oven dry) weight of wood digestively treated.

Generally it has been found that when the additive-towood ratio exceeds .01%, the pulps are perceptively lighter to the eye, and when treating resinous woods of the southern pine variety, observations of hand sheets prepared from the pulps show that the color tends to be yellower and have less red light reflecting constituents.

In sulphate mill operations which utilize tube type evaporators in their recovery system it is recommended that the additive-to-wood charging ratio be not greater than about 0.5% so as to avoid the danger of forming excessive amounts of scale therein. In broader aspects, however, the invention contemplates a digestive treatment of wood in sulphate cooking process liquor to which the additive material has been charged in an amount corresponding to an additive-to-wood charging ratio between about .01% and 1%.

Various aspects of the invention will be apparent from the data reported in Table I which was compiled from a series of pilot plant type runs designed to show the effect of using various amount of sodium metasilicate as a cooking liquor additive in sulphate process type digests of wood of the southern pine variety. The respective datum reported in each run is an average of the particular result obtained from the number of cooks indicated.

All cooks in all runs employed wood chips which had been screened to pass a 1 round holed screen and to be retained on a 1/2 round holed screen, and a charge of wood chips and cooking liquor sufficient to provide a chemical-to-wood charging ratio of 14% effective alkali based on the oven dry weight of wood charged. Furthermore, all cooks were carried out in a mild steel digester which was heated by live steam injection and automatically controlled during the digestive or cooking treatment by a cam operated pressure responsive device designed to provide a pressure rise period of 45 minutes to obtain a full pressure of 90 p.s.i.g. This pressure was thereafter maintained for 1 hour and the digester contents blown to a blow tank. The digester was automatically and uniformly relieved during the cooks so as to maintain a saturated steam temperature corresponding to the pres-V sure at all times and to expel non-condensable gases, evolved turpentine and other constituents normally expelled by such procedures. Thus, the temperature at full pressure was about 330 F.

Following the blow, the raw pulps were uniformly washed, screened on a .025" flat pulp screen, and the screened pulps refined in a Rapid Cycle Noble and Wood beater under conditions which were uniform in all cases. Pulp samples were taken at uniform intervals during the refinements for determining the drainage characteristics thereof, and hand sheets were also prepared from the respective pulps at various uniform intervals during their refinement for color, brightness and strength property evaluations.

The wood employed in Runs A, B, C and D was the same and was derived from fresh cut long leaf yellow pine logs, whereas the wood employed in Run E was derived from a slash pine log which had been cut more than three months prior to chipping and use, and therefore was considered to be old wood.

White cooking liquor derived from an actual kraft pulp mill was employed as the digesting liquor in all cooks, and all cooks in any one run employed a cooking liquor which was obtained at the same time from the kraft pulp mill and which had the same analysis. As between runs, however, the analysis of the cooking liquor was subject to the normal variations encountered in kraft mill operad tions. The white cooking liquors employed in the respective runs had the following analysis:

1 Grams per liter, as NaiO.

A technical grade of anhydrous sodium metasilicate, having a ratio of NaZOzSiOZ of 1:1 and an analyses of 51.0% Na2O, 45.5% SiO2 and 0.5% H2O by weight was employed in all cooks conducted in accord with the invention and was charged to the digester, as such, and in accord with the additive-to-wood charging ratio indicated in the table. The silicate was charged to the digester in each appropriate cook after the digester was loaded with chips and prior to the addition of the cooking liquor.

It is evident from the table that the additive caused an increase in brightness properties and facilitated the obtainment of pulps having lighter colors in all runs, and that the color and brightness properties were most significantly affected when the additive-to-wood ratio exceeded about .05%. On the other hand it has been found, as indicated by Run B, that the pulps produced by sulphate digests incorporating the additives in amounts corresponding to an additive-to-wood ratio in the range of from in excess of about .02% to about .05% are not only lighter and brighter but are much freer in the sense that their drainage characteristics (e.g. freeness value) are considerably better than the control pulps when the pulps compared have been refined to provide similar mullen strength characteristics. The fact that the pulps are freer in this range is important in mill operations because the pulps employed in producing paper are usually refined to give the desired mullen strength property, and the speed at which the paper machine is operated is mainly determined by the drainage characteristics which result from this refinement. Consequently, the freer pulps produced in accord with this phase of the invention enable the use of greater wire speeds whereby the capacity of a paper machine may be increased or provide greater drainage on the wire than would otherwise beexpected with a resultant saving in drier costs in the drying section of the paper machine.

In accord with other aspects of the invention it has also been found that the pulp produced in accord with the invention are stronger in the sense that the mullen, tear and drainage properties are more rapidly developed by refinement when the additive-to-wood charging ratios exceed about .02%. Consequently, power consumption for refining purposes can be decreased through practice of the invention in accord with this aspect thereof.

One aspect of the invention embodies the incorporation of the above findings in mill operations using the primary and secondary stock addition techniques to form composite sheets from pulps having different color and brightness properties. By employing an additive-to-wood charging ratio between about .02% to and about .05% when cooking wood to produce a high yield pulp for subsequent use as primary stock feed to the paper machine, the amount of lighter colored secondary stock employed in the formation of the composite sheets may be decreased because of the lighter colored properties imparted to the primary pulp which minimize the problem of color-showthrough. Furthermore, such use produces a freer pulp, which in addition to draining faster itself, also permits the water from the secondary pulp to drain more rapidly through it with the result that one is able to increase the speed and capacity of the paper machine. There is, of course, the further advantage that the primary pulp is stronger and requires less refining to develop the strength and drainage properties desired therein. In accord with this aspect of the invention the secondary stock may be advantageously derived in any suitable manner which produces a secondary pulp that is lighter and brighter than the primary pulp, and obviously when even better color properties are required of the primary pulps, greater amounts of the additive may be employed.

In accord with a preferred embodiment of the invention the primary pulp is derived from a high yield kraft type digest which employs one or more of the additives, and preferably sodium metasilicate, in amounts sumcient to provide an additive-to-wood charging ratio in the range of from in excess of about .02% to about .05%, and the secondary pulp is produced by a sulphate type digesting procedure which involves the use of a cooking liquor additive in accord with the invention and in amounts which exceed an additive-to-wood charging ratio of about .05%. In this way the color and brightness properties of both sides of the sheet are improved and less secondary stock is required to overcome the mottle effect. Furthermore, machine speeds may be increased by virtue of the better drainage characteristics of the primary stock, and both pulps are more readily refined because of their greater strength. In treating resinous woods of the southern pine variety it has been found that high yield pulps may be utilized as the secondary, as well as the primary, stock when they are produced in accord with the preferred aspect of the invention, and that the sheet obtained is competitively marketable with composite sheets derived through the use of low yield secondary stock.

In actual kraft mill operations conducted in accord with the preferred embodiment disclosed herein, it has been found as indicated in Run E of Table I that through the use of the additives, an actual increase in yield is realized when treating old wood such as employed in Run E of Table I. This is surprising and indeed beneficial because it is generally accepted by those skilled in the art that the use of old wood results in a substantial loss in the amount of pulp recovered therefrom. Consequently, these losses may be minimized by employing the digester additives in accord with the invention.

The invention is further illustrated by the following eX- amples.

Example I A top lled, upright batch type digester is charged with southern pine wood chips and a kraft mill cooking liquor which consists of white liquor and black liquor in a volume ratio of white liquor to black liquor to 5.l:1. The white liquor has a total alkali content of 100 grams per liter, an active alkali content of 86 grams per liter and a sulphidity of 31%. The amount of cooking liquor charged is sufficient to incorporate in the digester 10.8% effective alkali based on the oven dry Weight of wood charged. A commercial grade of anhydrous sodium metasilicate is also charged to the digester in amounts sufficient to provide an additive-to-wood ratio of .12% sodium metasilicate, as equivalent SOZ, on the basis of the oven dry wood. The sodium silicate additive in this case is sprayed on the chips as a by weight solution thereof in water.

Thereafter the digester is capped and the pressure and temperature uniformly elevated therein by live steam injection to 105 p.s.i.g. and 340 F. over a period of 54 minutes. Thereafter the cook is carried out at full pressure for 51 minutes, and blown to a blow tank. Turpentine and non-condensable gases are continuously bled off during the digest. The pulp is thereafter thoroughly washed and screened, and the total yield and screen yield determined. The screened pulp is then refined in a Rapid Cycle Noble and Wood beater, and the drainage characteristics, brightness values and strength properties determined at fixed time intervals of refining in a standard method. The silicate pulp may be compared with a control pulp derived by the same procedure except for the 1 Percent weight ratio of pulp yielded to wood charged to digester (oven dry basis).

2 Canadian standard.

3 Percent reflectance of 460 millimicron wave lengths as compared to Mg() standard white body having reflectance.

4 TAPPI Standard Bursting Strength, T403 rn-53 as corrected Aug. 1953 (lbs./in.2).

5 TAPPI Standard, T414 m-tg as corrected Sept. 1949 (grams).

5 Percent weight ratio of pulp passing a .025l i at pulp screen to wood charged to digester (oven dry basis).

Furthermore, the color of the silicate pulp will be considerably lighter to the eye than the control pulp.

Example II Total alkali 99.8 Effective alkali 72.5 Active alkali 85.7

The cooking procedure involved a 54 minute pressure rise period and a 126 minute full pressure period prior to the blow, the cook being carried out at a full pressure of p.s.i.g. and a corresponding saturated steam temperature. Anhydrous sodium metasilicate was charged to the batch type top filled digester after the chips had been added and prior to the addition of the cooking liquor and was charged in the amounts indicated.

Scrceupe Cook No. Ea 1 Si02/ Total Screen Pulp Wood,2 Yield 3 Yield 4 Brightpercent ness 5 16 .23 40. 6 38.1 30.0 18 23 39. 3 37.8 31.0 18 Control 39. 37. 5 30.0

1 Effective alkali, as NaQO, based on bone dry weight of wood.

2 Silicate-to-wood ratio.

aPercent: weight ratio of pulp yielded to wood charged to digestcr (oven dry basis).

4 Percent weight ratio of ulp passing a .025 flat pulp screen to wood charged to digester (oven ry basis).

5 Percent reflectance of 460 millimicron wave lengths as compared to M gO standard white body having 100% reflectance.

As is evident from the reported results the additive gave a brightness increase in Cook No. X comparable to an increase in the effective alkali from 16 to 18. The color properties of the pulp derived in Cook X were about the same as those of the pulp derived from Cook Z. The pulp from Cook Y was lighter in color than the pulp from Cook Z.

Example III Total alkali 115.8 Effective alkali 79.1 Active alkali 94.5

are charged to a digester in amounts sufficient to provide a chemical-to-wood charging ratio corresponding to an effective alkali charge of 1-"l%, based on the oven dry weight of wood. A commercial grade of anhydrous potassium metasilicate in an amount sufiicient to provide an additive-to-wood ratio of .07% is also added to the digester. Thereafter the digester is capped and live steam injected to uniformly elevate the pressure to 90 p.s.i.g. in 45 minutes, after which the cook is continued at this pressure for another hour, and blown to a blow tank. The pulp derived from the cook will be found to have better color and brightness characteristics than pulps produced without the employment of the additive. The pulp will also be stronger and develop its drainage properties more rapidly during refinement.

Example 1V Similar results may be obtained by substituting lithium metasilicate for the potassium metasilicate employed in Example lll.

A digesting procedure which may be followed in accord with the invention is illustrated by reference to FIG. 1, wherein 1 represents a top lled digester such as commonly employed in kraft pulp mill operations. rl`he digester is filled with wood chips of the southern pine variety as at 2, in an amount corresponding to a wood charge of 55,000 lbs. of oven dry wood. Thereafter 150 lbs. of anhydrous sodium metasilicate is charged to the digester as at 3. Next, 10,500 gallons of white cooking liquor is charged to the digester as at d, the cooking liquor containing 10,500 lbs. of total alkali, as Naz() 7,350 lbs. of eifective alkali, as Naz() and 8,000 lbs. of active alkali, as NaZO. Thereafter, the digester is capped, and live steam injected, as at 5, to heat the digester contents up to a temperature of 330 F. at a pressure of 90 p.s.i.g., a period of about 45 minutes being employed to obtain full pressure. Thereafter the pressure and temperature are maintained for a full pressure cooking period of 60 minutes and the contents of the digester then blown as at 6 to a blow tank to obtain a raw pulp. The period of the blow may take about l minutes. Throughout the cooking procedure the digester is relieved as at '7 to expel non-condensable gases, turpentine and the like. The amount of steam condensed within the digester during the cooking procedure may correspond to about 4,000 gallons of H2O. The raw pulp derived from the cook will be lighter in color, have better brightness characteristics, require less refining to develop its drainage and strength properties than a pulp produced in accord with the same procedure, but without the use of the silicate additive.

FIG. 2 schematically illustrates processing steps in a kraft type pulp and paper mill embodying certain aspects of the invention.

The illustration includes a wood preparation section, generally designated as 11, a wood pulping and refining section, generally designated as 12, a chemical recovery section, generally designated as 13, and a paper machine, which is generally designated as 14.

The Wood as received at the pulp mill is usually in the form of logs which are fed as illustrated by arrow to a barking drum 16. The bark is removed from the drum 16 as at 17, and barked logs are then conveyed, as indicated by line 18, to a chipper 19. The chips which are derived from chipper 19 are then passed to a chip storage bin 20 as indicated by line 21.

In accord with the illustration parallelly operated pulping and rening sections are maintained so as to separately process the wood chips into primary stock feed and into secondary stock feed for the paper machine 14, and thus enable the production of a paper sheet which is a composite of the pulps produced in the respective sections. The primary pulping and refining section is generally designated as 21, whereas the secondary pulping and rening section is generally designated as 22.

10 With reference to the primary section 21, chips from storage bin 20, and which are of the southern pine variety in accord with the illustration, are fed to batch type digester 23 as indicated by line 24, and white cooking liquor is fed thereto as indicated by line 25. The cooking liquor may have the following characteristics G./l., as NaZO Total alkali 119.8 Effective alkali 82.3 Active alkali 98.5

and is employed in an amount sufiicient to provide a charge of effective alkali to the digester of 13% based on the oven dry Weight of wood chips charged thereto. Anhydrous sodium metasilicate is also added, as indicated by arrow 2id, to digester 23, and in amounts sufficient to provide an additive-to-wood ratio of .03%, i.e. .03% sodium silicate, as equivalent SiOZ, based on the bone dry weight of wood to be digestively treated. Thereafter the contents of the digester are heated by live steam which is injected therein as indicated by arrow 27, the temperature and pressure being elevated to 330 F. and 90 p.s.i.g., respectively during a pressure rise period of 45 minutes. Thereafter the temperature and pressure are maintained at this level for an additional one hour period and the contents of the digester blown to blow tank 2S as indicated at 29. During the cooking treatment in digester 23, the digester is relieved and the turpentine evolved from the wood is removed as indicated by arrow 30 together with the non-condensable gases entrapped in the digester and other Volatile material.

The raw pulp which is produced by the cooking and blowing procedures is next passed to breaker 31, as indicated by line 32, wherein knots and poorly digested chips are broken up, the pulp then being passed, as indicated by line .33, to a disc type rener 34 wherein it is subjected to partial refinement prior to passing, as is indicated by line 35, to the pulp washers, that are generally illustrated at 36. The washers 36 may be the conventionally employed counter current drum type washers well known in the industry. The pulp from the Washers 36 which has been washed free of the spent cooking liquor is then passed, as indicated by arrow 37 to high density pulp storage tank 38, and the black liquor from the washers 36 is passed to black liquor tank 39 as indicated by arrowed line di?. From pulp storage tank 38 the partially refined pulp is passed as shown by line 41 to screen 42 wherein coarse material is removed as indicated by arrow 413. The screened pulp is then passed as shown by arrow 44 to another disc type reiiner i5 for further renement, and thence to cone type Jordan renner 46 as shown by line 47. Following the Jordans, the pulp is suitably diluted with water as shown by arrow 48 and passed as indicated by arrow 49 to the primary head box 50 of paper machine 14.

The overall process of transforming the wood chips into the secondary stock feed in the secondary pulping and refining section 22 is similar to the process carried out in the primary section 21, the principal differences residing in the degree of refining as when it is desired to produce a finished sheet on the secondary side thereof, and the amount of additive employed during digestion. Thus, wood chips from bin 20 are fed to digester 51, as indicated by arrow 52, and white liquor from white liquor storage tank 53 is fed to the digester as indicated by arrow 54. The cooking liquor employed is the same as that employed in the primary section and the amount of cooking liquor charged to digester 51 is sufficient to provide a charge of effective alkali to the digester of 13.5%, based on the oven dry weight of wood chips being treated.

Anhydrous sodium silicate is also added to the digester.

51, as indicated by arrow 55, and in amounts sulcient to provide an additive-to-Wood ratio of .15%, i.e. .15% sodium silicate, as equivalent SiOz, based on the bone dry weight of wood being digestively treated. The wood chips are then cooked in the digester by injecting steam therein, as indicated by arrow 56, to elevate the temperature and pressure to 330 F. and 90 p.s.i.g., respectively during a pressure rise period of 45 minutes. This is followed by a full pressure cooking period of 1 hour, after which the contents of the digester are blown to blow tank 57, as shown by arrow 58. During the cooking procedure, the digester is relieved as indicated by arrow 59 and the turpentine and non-condensable gases removed therefrom in the usual manner.

As shown by arrow 61, the raw pulp from blow tank 57 is next passed to breaker 60 which serves the same function in the secondary section as breaker 31 in the primary section. Thereafter the pulp passes to refiner 62, as illustrated by arrow 63 wherein it is subjected to a preliminary renement prior to its passage, as shown by line 64, to secondary pulp washers 65 wherein the partially refined pulp is washed free of its accompanying spent liquor. The black liquor from the washers 65 is passed to black liquor storage tank 39 as shown by line 66 and arrowed line 40 for treatment in the chemical recovery section 13.

The Washed pulp in the secondary section is next transferred to a pulp storage tank 67, as indicated by arrow 68, from whence it passes, as shown by arrowed line 69, to screen 70. Coarse material is removed from screen 70 as at 71, and the screened pulp is passed to disc type refiuer 72 as shown by line 73. Thereafter, the partly refined pulp from reiiner 72 passes, as shown by line 74 to a conical type Jordan refiner 75.

From Jordan 75 the refined pulp passes to secondary head box 76 as shown by line 77, and is diluted, as illustrated by line 78, to form a suitable secondary stock.

In forming a composite sheet of paper in accord with the invention a major amount of the stock added to the wire 79 of the paper machine which will be understood as traveling in the direction of arrow 79 is added as a primary stock addition, as illustrated by arrow 80, from primary head box 50, and a minor amount, in the range of from in excess of about 15% to about 25% of the total weight of stock added to the wire, is added as a secondary stock addition on top of the primary stock from secondary head box 76, as shown by arrow 81. As is apparent to those skilled in the art, a portion of the water in the stock added to the wire 79, drains through and is collected in wire pit 82 from whence it is removed and usually employed for dilution purposes.

The web of paper 85 which is formed on wire 79 is thereafter treated in the usual manner, being pressed, dried, calendered and rolled onto a reel. Thus, the web 85 may be picked up from the wire, as at S3 by press felt 84, and thereafter suitably pressed between rollers 86 and then transferred to another press felt 87 and further pressed between rollers such as rollers 88 and 89. The sheet is then dried in the drying section of the paper machine by passing over heated rolls such as illustrated at 90, bottom and top drier felts 100 and 101, respectively being employed to aid in the drying process. Thereafter the sheet is passed to the calender section generally shown at 102, and subsequently wound on reel 103 as a finished sheet.

The black liquor from the washers 36 and 65 is collected in black liquor storage tank 39 and thereafter treated in the chemical recovery section 13. In this section 13 the black liquor passes as by line 104 to multiple e'ect tube type evaporators, and is concentrated therein by the evaporation of water to form a heavy black liquor which is passed from the last effect 106 to heavy liquor storage tank 107 as shown by line 108. At an intermediate stage in the evaporation process, the partially concentrated liquor is usually collected in a tank, such as tank 109, wherein the soap floats to the surface of the liquor and is skimmed olf as indicated by arrow 110. As previously stated, to avoid the danger of scale formations in the evaporators, an additive-to-Wood ratio which is less than about .5% is recommended in plants which eniploy tube type evaporating apparatus so as to avoid scale problems which may develop therein.

The heavy liquor is next passed as shown by line 111 to direct contact evaporator 112 wherein it is further concentrated by direct contact with the hot iiue gases obtained from furnace 113 as by line 114. The concentrated black liquor from evaporator 112 is then passed as shown by arrow 115 to furnace 113 and burned therein to form a molten smelt which is removed, as indicated by line 116, and passed to a dissolving tank 117. Salt cake may be also added to the furnace, as indicated by line 118 to supply make-up chemical for the plant. Experience in an actual kraft mill operated in accord with the invention has shown that salt cake requirements are decreased through the use of sodium silicate as an additive, thereby clearly indicating the make-up chemical function of sodium silicate additives.

In the dissolver 117 the smelt is leached with weak liquor from clarifier 119, as shown by line 120, to form green liquor which is then passed, as by line 121, to the clarifier unit 119 from which the dregs are removed as at 122, and the clarified green liquor passed, as indicated by line 123, to green liquor storage tank 124. The green liquor from tank 124 is next passed, as by line 125, to a slaking unit 126 wherein it is mixed with fresh make-up lime and with burnt lime from tank 127, as shown by lines 128 and 139, respectively. From the slaker 126 the liquor is passed to classifier 130, as by line 131 to remove nondispersed solids as at 134, and thence to the causticizer unit 132, as by line 133, wherein the green liquor is transformed to white cooking liquor which contains suspended lime mud. The liquor from unit 132 is then passed to a clarification unit 135, as by line 136, wherein the lime mud is separated and removed as indicated by line 137 and the clarified white cooking liquor is passed by line 138 to white liquor storage tank 53 for use in subsequent cooks.

In recovering the lime from the lime mud obtained from clarification unit 135, the mud is washed in mud washer 139, passed to mud thickener 140, as by line 141, from whence the thickened mud is transferred to filter 142, as by line 143. As shown, the overflow 147 from thickener 140 may be employed for washing the mud in washer 139, the overow from washer 139 being expelled therefrom as at 148. After being filtered in filter 142 the lime mud, which is principally calcium carbonate, is passed, as shown by line 144, to lime kiln 145 wherein it is burned to CaO and thereafter passed as shown by line 146 to burnt lime storage tank 127 for subsequent use in slaker 126.

It will be apparent that fillers and the like may also be incorporated in the pulps in the manner customarily employed in mill operations.

Provision is made for recycling black liquor as a diluent for the white liquor such as is customary in kraft mill operations. Thus, black liquor from tank 39 may be passed to digesters 23 and 51, respectively by means of line 149 and lines 25 and 54, respectively. It is preferred, however, that the wood digests be conducted by using a cooking liquor which is free of a black liquor component. Generally it has been found that through the dilution of the white liquor with black liquor that the color and brightness advantages of the invention are suppressed to some extent and, accordingly, not as fully realized as when the cooking liquor consists solely of white liquor. Nevertheless these advantages are obtained, although usually to a lesser extent, by sulphate process type digests employing the additive materials of the invention when the cooking liquor contains a black liquor component and, accordingly, the invention is not to be construed as limited to sulphate process type digests employing white liquor as the sole component of the cooking liquor unless expressly provided for in the claims.

From the foregoing it is evident that sulphate type wood pulps having improved color and brightness characteristics sass/,17a

may be obtained by pulping methods which provide for the digestive treatment of the wood in sulphate cooking liquor to which has been added a small amount of a material selected from one or more of the inorganic compounds in the group consisting of the alkali metal silicates and silicic acid wherein the amounts of such material added exceed an additive-to-wood charging ratio of .01%. Pilot plant experience has shown that these characteristics of the pulps are not suiiiciently inliuenced by the additives to warrant, as a practical matter, the use of the additives in amounts which exceed an additive-to-wood charging ratio of about 1.0%. Generally the pulps produced become lighter and brighter as greater amounts of the additives are employed in the sulphate type cooking procedures. However, experience has shown in kraft type digests of southern pine woods, as previously indicated, that such an appreciable and beneficial change in the color and brightness properties of the pulps develop as greater amounts of sodium metasilicate or sodium orthosilicate are employed in the area of an additive-to-wood ratio of about .05% as to warrant the conclusion that additive-to-wood charging ratios in excess of about .05% should be employed when seeking pulps having the best brightness and color characteristics.

lt should be understood that such terms as effective alkali, total alkali, active alkali and sulphidity are employed herein in accord with TAPPI standards for the Sulphate Pulping Process, 0400 p-54 as revised l une 1954, and that effective alkali, total alkali and active alkali are reported in terms of their equivalent Na2O in accord with such standards. Sulphidity is reported herein as the percentage ratio of NagS, expressed as Nago, to active alkali.

It should also be understood that brightness values reported herein were taken from handsheets by using a standard photovolt tester, and indicate the percent reflectance of 460 millimicron wave lengths wherein magnesium oxide is the standard 100% reilectance value.

Generally it is contemplated that the various beneiits of the invention may be obtained by treating both hard and soft woods. However, the inventive concept is particularly useful when treating resinous woods, such as those of the southern pine variety, because pulps derived from such woods are usually much darker and contain large amounts of resinous material which adversely affect the color and brightness properties of the pulps derived therefrom. Typical woods of the southern pine variety are slash pine, short-leaf pine, loblolly pine and long leaf pine.

The invention has been most particularly described herein as an improvement in kraft type processes. However, the additives may be employed in other sulphate type processes. For example, the inventive concept may be utilized in sulphate processes directed to obtainable bleachable grades of sulpate pulp, and as shown in Table I, it has been found that lower chlorine numbers are generally obtained when the additive-to-wood ratios exceed about .05%. Consequently, amounts in excess of this ratio are recommended when digesting conditions are directed to the obtainment of bleachable grades of sulphate pulps.

It may be mentioned at this point that attempts to obtain similar results by the use of the additives in soda cooks have met with no success.

WhileV only certain preferred embodiments of this invention have been shown and described by way of illustration, many modiiications will occur to those skilled in the art and it is, therefore, desired that it be understood that it is intended in the appended claims to cover all such modifications as fall within the true spirit and scope of this invention.

What is claimed as new what it is desired to secure by Letters Patent of the United States is:

1. In a sulphate wood pulping process, the improvement comprising digestively treating wood in sulphate process 14 cooking liquor to which has been added a small amount of material selected from members of the group consisting of silicic acid and the alkali metal silicates, said material being added in an amount between about .0l and about 1% by weight as equivalent SiO2, based on the oven dry weight of wood digestively treated.

2. In a sulphate process of producing wood pulp wherein wood particles are digestively treated in cooking liquor, the method of improving the color and brightness characteristics of the pulp comprising adding to the cooking liquor a small amount of material selected from members of the group consisting of silicic acid and the alkali metal silicates, said material being added in an amount between about .01% and about 1% by weight as equivalent SiOZ based on the oven dry weight of wood particles digestively treated.

3. In a sulphate process of producing wood pulp wherein resinous wood particles are digestively treated in cooking liquor, the method of producing freer pulp having irnproved color and brightness characteristics comprising adding to the cooking liquor a small amount of material selected from members of the group consisting of silicic acid and the alkali metal silicates, said material being added in an amount between about .002% and about .05 by weight as equivalent SiOZ, based on the oven dry weight of wood particles digestively treated.

4. In a sulphate process of producing wood pulp wherein resinous wood particles are digestively treated in sulphate process cooking liquor, the method of producing stronger pulp having improved color and brightness characteristics comprising adding to the cooking liquor a small amount of material selected from members of the group consisting of silicic acid and the alkali metal silicates, said material being added in an amount between about .02% and about 1% by weight as equivalent SiOz, based on the oven dry weight of wood particles digestively treated.

5. In a sulphate process of producing wood pulp from resinous wood of the southern pine variety comprising steps of digestively treating particles of the wood in sulphate process cooking liquor, and concentrating spent cooking liquor derived from the digestive treatment step in a tube type evaporator, the improvement comprising steps of adding a material selected from members of the group consisting of sodium orthosilicate and sodium metasilicate to said sulphate process cooking liquor and subjecting said particles of wood to the digestive treatment therein, said material being added to said sulphate process cooking liquor in a small amount between about .01% and about .5% by weight as equivalent Si02, based on the oven dry weight of the wood particles digestively treated therein, thereby to improve the color and brightness characteristics of the produced pulp while minimizing scale buildup in the tubes of said evaporator.

6. In a process of producing paper sheet from wood comprising steps of pulping a major portion of said wood by a sulphate pulping process that includes digestively treating particles of said major portion in sulphate process cooking liquor, adding a stock of one pulp obtained from the pulping of said major portion to the drainage wire of a paper machine, and adding a stock of another pulp derived from the remaining portion of the wood to the drainage wire of said paper machine on top of the one pulp added thereto, thereby to initially form said paper sheet as a web composed of said one pulp and the other pulp, the improvement comprising steps of adding a material selected 'from members of the group consisting of sodium orthosilicate and sodium metasilicate to said sulphate process cooking liquor, and subjecting the particles of said major portion of said wood to the digestive treatment therein, said material being added to said sulphate process cooking liquor in a small amount between about .02% and about .05% by weight as equivalent S102, based on the oven dry weight of the wood particles digestively treated therein, thereby to improve the drainage of water from the pulps added to said drainage wire.

7. In a process of producing paper sheet from wood comprising steps of adding a stock of one pulp derived from a major portion of said wood to the drainage wire of a paper machine, pulping the remaining portion of said wood by a sulphate wood pulping process that includes digestively treating particles of the remaining portion in sulphate process cooking liquor, adding a stock of another pulp obtained from the pulping of said remaining portion to the drainage wire of said paper machine on top of the one pulp added thereto, thereby to initially forrn said paper sheet as a web composed of said one pulp and said other pulp, the improvement comprising steps of adding a material selected from members of the group consisting of sodium orthosilicate and sodium metasilicate to said sulphate process cooking liquor, and subjecting said particles of said remaining portion to the digestive treatment therein, said material being added to said sulphate process cooking liquor in a small amount between about .05% and about 1% by Weight, as equivalent SiO2, based on the oven dry weight of the wood particles digestively treated therein, thereby to improve the color and brightness characteristics of the other pulp.

8. A process of producing paper sheet from southern pine wood comprising steps of adding a first material selected from members of the group consisting of sodium orthosilicate and sodium metasilicate to a first sulphate process cooking liquor, pulping a major portion of said wood by a sulphate wood pulping process that includes cooking said major portion in said first sulphate process cooking liquor, adding a second material selected from members of said group to a second sulphate process cooking liquor, pulping the remaining portion of said wood in said second sulphate process cooking liquor, refining and forming a first stock of pulp from the product of pulping said major portion, refining and forming a second stock of pulp from the product of pulping said remaining portion, adding said first stock of pulp to the drainage wire of a paper machine, and adding said second stock of pulp to the drainage wire of said paper machine on top of the pulp of said first stock, thereby to initially form said sheet as a web composed of pulps derived from said first stock and said second stock respectively, said first material being added to said first sulphate process cooking liquor in a small amount between about .02% and about .05% by weight, as equivalent SiOZ, based on the oven dry weight of said major portion subjected to the cooking in said first sulphate process cooking liquor, thereby to improve drainage of water from the pulps added to said drainage wire, said second material being added to said second sulphate process cooking liquor in a small amount between about .05% and about 1% by weight as equivalent SiO-2, based on the oven dry weight of said remaining portion subjected to the cooking in said second sulphate process cooking liquid, thereby to improve the color and brightness characteristics of the pulp of said second stock and of the produced paper sheet.

9. A process of producing paper sheet from wood of the southern pine variety comprising adding a first material selected from members of the group consisting of sodium orthosilicate and sodium metasilicate to a rst kraft process cooking liquor, adding a second material selected from members of said group to a second kraft process cooking liquor, pulping a major portion of said wood by a kraft wood pulping process that includes a first high yield digestive treatment of said major portion in said first kraft process cooking liquor, pulping the remaining portion of said wood by a kraft wood pulping process that includes a second high yield digestive treatment of said remaining portion in said second kraft process cooking liquor, refining and forming a first stock of pulp from the product of pulping said major portion, refining and forming a second stock of pulp from the product of pulping said remaining portion, adding said first stock of pulp to the drainage wire of a paper machine, and adding said second stock of pulp to the drainage wire of said paper machine on top of the pulp of said first stock, thereby to initially form said sheet as a web composed of pulps derived from said first stock and said second stock respectively, said first material being added to said first kraft process cooking liquor in a small amount between about .02% and about .05% by weight, as equivalent SiO2, based on the oven dry weight of said major portion subjected to said first high yield digestive treatment, thereby to improve drainage of water from the pulps added to said drainage wire and to improve the color and brightness characteristics of the pulp of said first stock and of the produced paper sheet, said second material being added to said second kraft process cooking liquor in a small amount between about .05% and about .5% by weight as equivalent SiOZ, based on the oven dry weight of said remaining portion of said wood subjected to said second high yield digestive treatment, thereby to improve the color and brightness characteristics of the pulp of said second stock and of the produced paper sheet.

10. In a sulphate Wood pulping process, the improvement comprising digestively treating wood in sulphate process cooking liquor to which has been added a small amount of material selected from members of the group consisting of sodium orthosilicate and sodium metasilicate, said material being added in an amount between about .01% and about 1% by weight as equivalent SiOz, based on the oven dry weight of wood digestively treated.

11. In a sulphate process of producing wood pulp wherein wood particles are digestively treated in cooking liquor, the method of improving the color and brightness characteristics of the pulp comprising adding to the cooking liquor a small amount of material selected from members of the group consisting of sodium orthosilicate and sodium metasilicate, said material being added in an amount between about .01% and about 1% by weight as equivalent SiO2, based on the oven dry weight of wood particles digestively treated.

12. In a sulphate process of producing wood pulp wherein resinous wood particles are digestively treated in cooking liquor, the method of producing freer pulp having improved color and brightness characteristics comprising adding to the cooking liquor a small amount of material selected from members of the group consisting of sodium metasilicate and sodium orthosilicate, said material being added in an amount between about .02% and about .05% be weight as equivalent SiOz, based on the oven dry weight of wood particles digestively treated.

13. In a sulphate process of producing wood pulp wherein resinous wood particles are digestively treated in sulphate process cooking liquor, the method of producing stronger pulp having improved color and brightness characteristics comprising adding to the cooking liquor a small amount of material selected from members of the group consisting of sodium metasilicate and sodium orthosilicate, said material being added in an amount between about .02% and about 1% by weight as equivalent SiO2, based on the oven dry weight of wood particles digestively treated.

References Cited in the file of this patent UNITED STATES PATENTS 1,775,405 Neumann Sept. 9, 1930 2,286,942 Tuten June 16, 1942 2,632,736 Currie Mar. 24, 1953 2,881,672 Thomas Apr. 14, 1959 `,054,717 Turner tsept. 18, 1962 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3, 147, 178 September l, 1964 Frank JQSowa It is hereby certified that error appears vin the above numbered patent requiring correction and that the said Letters Patent should read as corrected below.

Signed and sealed this 16th day of March l965 (SEAL) Attest:

ERNEST w. swIDERy EDWARD J. BRENNER Attesting Officer I v Commissioner of Patents 

1. IN A SULPHATE WOOD PULPING PROCESS, THE IMPROVEMENT COMPRISING DIGESTIVELY TREATING WOOD IN SULPHATE PROCESS COOKING LIQUOR TO WHICH HAS BEEN ADDED A SMALL AMOUNT OF MATERIAL SELECTED FROM MEMBERS OF THE GROUP CONSISTING OF SILICIC ACID AND THE ALKALI METAL SILICATES, SAID MATERIAL BEING ADDED IN AN AMOUNT BETWEEN ABOUT .01% AND ABOUT 1% BY WEIGHT AS EQUIVALENT SIO2, BASED ON THE OVEN DRY WEIGHT OF WOOD DIGESTIVELY TREATED. 